With in vivo and in vitro preparations of choroid plexus, the nature of transport of chloride and bicarbonate at the blood-cerebrospinal fluid barrier will be investigated in immature and mature rats (1-5 wk postnatal). Changes in the concentration of chloride and bicarbonate and hydrogen ion in the choroidal epithelium will be induced by altering the concentration of chloride and bicarbonate in the CSF medium (in vitro) and in the plasma in the in vivo experiments. Intracellular/extracellular gradients for chloride and bicarbonate will be determined under conditions of carbonic anhydrase inhibition with acetazolamide, and with the stilbene derivatives SITS and DIDS. Changes in choroid cell pH will also be quantitated in rats subjected to acute respiratory acidosis. With respect to both the immature choroid plexus and brain, the assessment of 1) the role of the carbonic anhydrase system in regulating bicarbonate concentration in the choroid plexus-CSF system, and 2) the buffering capacity of choroid plexus in animals ventilated with carbon dioxide, should help to elucidate the nature of acid-base regulation in the incompletely developed central nervous system.